I think he is against Civil Unions, but at one time a Constiutional Amendment was being thrown around that banned Gay Marraige and established Civil Unions because it was believed that it would get more support in the legislature. So Romney backed the Amendment. That amendment ended up failing, and he then proposed the current amendment that is up for a vote that bans Gay Marriage only.
Mitt's current position on protecting marriage is a flip-flop itself.
In seeking the homosexual Log Cabin Republican endorsment (which he got twice), he said: “The authorization of marriage on a same-sex basis falls under state jurisdiction.”
In other words, it's up to each state to decide, identical to McCain's argument against a federal marriage amendment.
Running for governor in 2002, Romney publicly OPPOSED a state Marriage Protection Amendment proposed before his state Supremes legalized so-called homosexual "marriage," calling it "too extreme" -- even though his wife and son signed the petition to put it on the ballot.
But hey, Mitt's no dummy. After getting elected governor, he put that presidential campaign finger up in the air and correctly calculated that he could get some national mileage by making himself out to be the crusading champion of marriage amendments.